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Biomedical subjects

R F Newbold

Publications and source records attributed to R F Newbold.

At least 55 records · Page 3Linked to original sources
PubMed1996

Identification of human tumour suppressor genes by monochromosome transfer: rapid growth-arrest response mapped to 9p21 is mediated solely by the cyclin-D-dependent kinase inhibitor gene, CDKN2A (p16INK4A).

Microcell transfer of intact normal human chromosomes into immortal mouse and hamster fibroblast cell lines has revealed growth suppressive activity associated with a small sub-set of the human complement. Here, we describe the results of a detailed study aimed at identifying the gene or genes responsible for the rapid growth-arrest response obtained with human chromosome-9. Initially, STS-PCR deletion mapping of segregants arising in monochromosome transfer experiments was used successfully to localize the active sub-chromosomal region to 9p21. Subsequent fine-structure deletion mapping of previously uniformative hybrid segregants, employing additional markers between D9S162 and D9S171, provided strong evidence that the cyclin-dependent kinase (cdk) inhibitor gene CDKN2A (p16INK4A) was solely responsible for the chromosome-9 effect; 9p21 microdeletions in a significant proportion of segregant clones were restricted to a single CDKN2A exon. Transfection experiments with CDKN2A and CDKN2B cDNA expression vectors, using mouse A9 cells and three human malignant melanoma cell lines as recipients, provided further evidence in support of this hypothesis. Collectively, our results indicate that expression of human CDKN2A (controlled either by its natural regulatory elements, or by a cytomegalovirus promoter) is incompatible with in vitro proliferation in immortalized rodent cells and in human melanoma cell lines. The rapidity of the growth inhibitory effects of CDKN2A was inconsistent with a mode of action involving induction of replicative cell senescence via telomerase repression, but was consistent with a mechanism based on cell cycle arrest through cdk inhibition. The study described here has generated a panel of microdeleted monochromosome-9 donor hybrids which may prove valuable in functional investigations aimed at identifying other important tumour suppressor genes located on human chromosome-9.

Animals↗
PubMed1995

Novel use of a selectable fusion gene as an "in-out" marker for studying genetic loss in mammalian cells.

Recent demonstrations of loss of heterozygosity in a wide variety of human cancers suggest that large multilocus genetic deletions (presumably including tumor suppressor genes) constitute a major class of genetic alteration in human carcinogenesis. Here we show that a bifunctional fusion gene (Hytk), suitable for both positive and negative selection, is an effective marker for studying genetic loss in mammalian cells with minimal interference from point-mutational changes. Studies with a transgenic V79 cell line in which a single functional copy of Hytk was stably inserted into the genome in a retroviral vector showed that loss of the marker (and presumably flanking cellular genetic material) could be induced efficiently by ionizing radiation (gamma-rays and fast neutrons) but only weakly by the powerful point-mutagen benzo[a]pyrene diol epoxide. In a first application of the system, we provide evidence that radiation-induced loss can occur through an indirect mechanism after a high-frequency event. Collectively, our results suggest that the Hytk marker should be a valuable tool for studying genome position effects on the tolerance of genetic loss in cultured human cells that represent different stages in clonal evolution and tumor progression.

3T3 Cells↗
PubMed1995

Mechanisms involved in the immortalization of mammalian cells by ionizing radiation and chemical carcinogens.

Immortalization is a prerequisite for the clonal evolution and malignant transformation of normal mammalian cells in culture. In order to gain a mechanistic insight into the genetics of carcinogen-induced cellular immortality, a cell culture assay has been developed based on the use of freshly explanted Syrian hamster dermal (SHD) fibroblasts. The relative efficacies of a variety of chemical and physical carcinogens at immortalizing SHD cells (against a zero background of spontaneous immortalization) were compared. Ionizing radiation and nickel chloride appeared to be more effective as immortalizing agents than powerful point mutagens, suggesting (but not proving) that clastogenic damage may be more significant in the immortalization process than point mutation. Frequencies of induced immortality (10(-6)-10(-7)/treated cell) were arguably consistent with a direct mutational mechanism involving a single genetic target. However, detailed cytogenetic characterization of a panel of newly immortalized cell lines revealed no non-random chromosomal alterations in the cells at the level of G-banding. Furthermore, additional experiments with the SHD system have provided confirmatory evidence that immortalization can occur as an indirect consequence of carcinogen exposure following an induced high frequency change in the treated population, rather than through direct targeted mutagenesis. Previous somatic cell genetic studies have suggested the possibility that a target gene for immortalization exists on the human and Chinese hamster X chromosomes. Here we provide strong evidence that the normal SHD X chromosome displays powerful senescence-inducing properties when introduced, by microcell transfer, into newly immortalized SHD recipients. These results suggest that induction of the immortal phenotype in SHD cells by carcinogens results primarily from functional inactivation of a senescence gene which may be X-linked. One possible mechanism for senescence gene inactivation consistent with our observations is through a sub-microscopic interstitial genetic deletion. However, the considerable efficacy of nickel (a human carcinogen) as an immortalizing agent at nonmutagenic doses raises the alternative possibility that immortalization may occur through an epigenetic mechanism.

Animals↗
PubMed1995

Construction and characterization of a highly stable human: rodent monochromosomal hybrid panel for genetic complementation and genome mapping studies.

Human:rodent somatic cell hybrids carrying a single, intact, selectable human chromosome are valuable both for functional somatic cell genetic analysis and genome mapping procedures. Here, we describe the construction and detailed molecular cytogenetic characterization of a panel of 23 stable hybrids, representing all 22 human autosomes plus the X-chromosome. Individual normal human chromosomes have been tagged with a selectable fusion gene (Hytk) introduced into the chromosome in a small (4.2 kbp) retroviral vector. Use of the Hytk marker permits both positive and negative ("in-out") selection to be applied to the human chromosome in any mammalian cell background. The panel includes 18 new hybrids isolated by direct microcell transfer from normal human diploid fibroblasts into mouse A9 cells.

Adult↗
PubMed1993

Cell immortalization as a key, rate-limiting event in malignant transformation: approaches toward a molecular genetic analysis.

Recent advances using somatic cell genetic approaches have provided a convincing body of evidence that the senescence of mammalian cells in culture is controlled by a small group of genes, one or more of which are functionally deleted in the process of immortalization. Microcell-mediated mono-chromosomal transfer methods should permit precise mapping of these genes to specific chromosomal regions. Cloning of senescence genes, using either conventional 'positional cloning' techniques or retroviral insertion mutagenesis, is now a realistic possibility. The leap in our understanding of the molecular genetic events driving the alternative cellular states of limited proliferative capacity and immortality, which such advances should precipitate, will finally permit the question of the role of cell immortalization in cancer to be addressed, and may open the door to the design of new modes of cancer therapy. In addition, the precise mechanism underlying the wide difference in transformability between human and rodent cells, which should also emerge from these investigations, is likely to make a significant contribution towards resolving the key issue of the relevance of rodent tumour induction assays in assessing the potential carcinogenicity of environmental chemicals.

Animals↗
PubMed1988

Morphological transformation of immortalized hamster dermal fibroblasts following treatment with simple alkylating carcinogens.

We have examined the mechanism of transformation of a line of immortalized hamster dermal fibroblasts (4DH2 cells) following treatment with the simple alkylating agents, N-methyl-N-nitrosourea (MNU), N-ethyl-N-nitrosourea (ENU) and dimethyl sulphate (DMS). Treatment of 4DH2 cells with the potent point mutagens MNU and ENU gave rise to a spectrum of foci of different sizes, including progressively growing large foci and compact small foci. In contrast, treatment with the weak point mutagen DMS produced mostly large foci. The ability of cell lines derived from morphologically transformed foci to grow in soft agar in general reflects their original size. Thus most cell lines derived from large foci grew in soft agar while most lines derived from small foci did not. Transfection of cellular DNAs into the parent 4DH2 cell line and into NIH3T3 mouse fibroblasts has revealed the presence of dominantly acting transforming genes in the chemically transformed cell lines. Thus DNA from five of six cell lines derived by culturing large foci and from one of three cell lines derived by culturing small foci induced efficient morphological transformation of the recipient cells. Southern analyses of DNA from primary and secondary transfectants showed that several of the transforming genes transferred in these experiments were not closely related to H-ras, K-ras or N-ras.

Alkylating Agents↗
PubMed1982

Biochemical and immunological characterisation of mutants induced in V79 Chinese hamster cells by a benzo[a]pyrene diolepoxide.

A series of 8-azaguanine resistant mutants was induced by treatment of V79 Chinese hamster cells with either r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (antiBPDE) or methylnitrosourea (MNU). Hypoxanthine phosphoribosyltransferase (HPRT) activity in the mutants was determined for both hypoxanthine and azaguanine as substrates. With antiserum to purified brain HPRT, cross-reacting material was also determined and analysed by two dimensional polyacrylamide gel electrophoresis. By these criteria mutants induced by anti-BPDE or MNU did not differ appreciably and the data obtained was consistent with the induction of point mutations by both carcinogens. The relevance of these results to the correlation of carcinogenicity with mutagenicity in V79 cells, but not in bacteria, is discussed.

7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide↗
PubMed1981

Inhibition of metabolic cooperation between mammalian cells in culture by tumor promoters.

The influence of phorbol-related tumour promoters and non-promoters on metabolic cooperation between wild-type and mutant Chinese hamster cells has been studied. The recovery, in medium containing 8-azaguanine, of hypoxanthine phosphoribosyl transferase-deficient (HPRT-) V79 cells co-cultured with an excess (2 x 10(6) per 9 cm petri-dish) of wild-type cells was determined in the presence and absence of each compound. Under the latter conditions (solvent treatment only) metabolic cooperation consistently reduced the cloning efficiency of HPRT- cells to approximately 10% of that in cultures without wild-type cells. However, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), the most potent known tumour promoter, almost totally reversed the effect of wild-type cells on mutant recovery when included in the medium at concentrations as low as 1 nM. TPA also markedly enhanced the expression, in crowded cultures, of HPRT- mutants induced by the carcinogen N-methyl-N-nitrosourea. This property was not shared by other phorbol esters which are inactive as tumour promoters, or by polycyclic aromatic hydrocarbons, but was exhibited to a lesser degree by mezerein, a diterpene ester of significant but weaker promoting activity than TPA. Confirmation that the effect of TPA on mutant expression is the result of inhibition of metabolic cooperation was obtained in experiments using autoradiography, which showed that low doses are able to block the transfer of [3H]-uridine nucleotides from prelabelled V79 cells to unlabelled V79 cells in contact. These findings have prompted us to formulate a working hypothesis for the mode of action of TPA in vivo as a tumour promoter based on its interference with this type of intercellular communication.

Carcinogens↗
PubMed1980

Benzo(a)pyrene metabolism and DNA adduct formation in serially cultivated strains of human epidermal keratinocytes.

Six strains of human epidermal keratinocytes were shown to be capable of metabolising benzo(a)pyrene (BP)for at least 40 population doublings in culture. Metabolism was independent of human or mouse fibroblast products, and also the growth rate of the epidermal cultures, at least when cholera toxin and epidermal growth factor were included in the medium. Epidermal strains metabolized and bound to their DNA more BP than human fibroblasts, and themselves exhibited a 2-to 3-fold range of inter-strain variability. Chromatographic analysis of epidermal DNA containing bound BP showed that the major metabolite which had reacted with the DNA was a 7,8 dihydrodiol - 9,10 oxide of BP (the proposed ultimate carcinogenic metabolite of BP).

Animals↗
PubMed1980

Mutagenicity of carcinogenic methylating agents is associated with a specific DNA modification.

The carcinogenic potency of simple aliphatic alkylating agents such as the alkylnitrosamides and the alkylmethanesulphonates is positively correlated with their ability to alkylate the relatively weakly nucleophilic oxygen atoms in DNA, particularly the O6-atom of guanine. Differences in the spectrum of DNA alkylations produced by these agents can be rationalised on chemical grounds in that the electrophilic reactivity of the alkylating species determines the extent to which it will react at sites of weaker nucleophilicity. Alkylation of the more strongly nucleophilic ring nitrogen atoms of the purine bases, which is the main site of reaction with all these agents, appears to be much less important in alkylation carcinogenesis. O6-alkylation of guanine is likely to interfere with DNA base-pair hydrogen bonding and is possibly the major DNA modification responsible for the induction of GC yields AT transition mutations in bacteria and bacteriophage by alkylating agents. Here, we have studied the effects of three methylating agents of contrasting carcinogenic potency on mammalian (V79 Chinese hamster) cells in in culture. We report that the mutagenicity but not the cytotoxicity of each agent reflects its carcinogenicity and, furthermore, that the marked differences in mutagenicity are closely paralleled by differences in levels of O6-guanine methylation.

Alkylating Agents↗
PubMed1979

Cell-mediated mutagenesis in cultured Chinese hamster cells by polycyclic hydrocarbons: mutagenicity and DNA reaction related to carcinogenicity in a series of compounds.

Three polycyclic hydrocarbons, benz(a)anthracene, 3-methylcholanthrene and 7,12-dimethylbenz(a)anthracene, have been studied in a cell-mediated mutagenesis system using BHK 21 cells to metabolize the hydrocarbons and V-79 cells as targets for detecting induced cytotoxicity and mutation. In large-scale experiments, the DNA of V-79 cells was analyzed by column chromatography to determine the nature and true extent of reaction of hydrocarbons with dexoyribonucleosides. Products with DNA formed by the two carcinogenic compounds were qualitatively very similar to those reported to occur in vivo and in primary cell cultures. Binding indices were calculated from the tritium content of DNA-hydrocarbon products, related to overall metabolism, for these two compounds together with benzo(a)pyrene and 7-methylbenz(a)anthracene using data from a previous study. These values reflected differences in carcinogenic potency between the compounds. Induced mutation frequencies were related to the extent of DNA reaction with each compound. At equivalent extents of DNA reaction with hydrocarbon products, levels of induced mutation were not significantly different.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1979

A quantitative comparison of the mutagenicity of carcinogenic polycyclic hydrocarbon derivatives in cultured mammalian cells.

The mutagenicity of a series of reactive polycyclic hydrocarbon derivatives has been studied using Chinese hamster (V79) cells in culture and, as a mutational marker, resistance to the purine analogue 8-azaguanine. The compounds were compared by relating mutation frequency to the dose applied (mutagenic effectiveness) to induced cytotoxicity (mutagenic efficiency) and to the extent of reaction of the hydrocarbon with DNA (absolute mutagenic efficiency). In each case anti-benzo(alpha)pyrene (BP)-7,8 dihydrodiol-9,10 oxide, the suspected ultimate carcinogenic form of benzo(alpha)pyrene, was by far the most potent of the compounds tested. Furthermore, the mutagenicity of the syn- and anti-BP-diolepoxide isomers correlated positively with their documenrences in the ability of each derivative to form a carbonium ion. Variations in mutagenic efficiency and absolute mutagenic efficiency were more difficult to explain. The latter findings are discussed in relation to the types of hydrocarbon-DNA product obtained with each compound and also to the possibility of a variable cellular response to more subtle differences in the chemistry of the hydrocarbon-DNA interaction.

Animals↗
PubMed1979

The cytotoxic, mutagenic and clastogenic effects of chromium-containing compounds on mammalian cells in culture.

Examples of chromic and chromate salts have been examined for their effects on a cultured Chinese hamster cell line. The responses studied were cytotoxicity, mutagenesis and clastogenesis. Chromate (hexavalent chromium) salts of both high and medium water solubility were active in producing all three classes of response, whereas an insoluble chromate salt and a soluble chromic (trivalent chromium) salt were inactive. In addition to illustrating the value of using mammalian cells in culture for screening chemicals for biological activity, the results of this study reinforce current views regarding the genotoxic properties of chromates.

Animals↗
PubMed1977

Cell-mediated mutagenesis in cultured Chinese hamster cells by carcinogenic polycyclic hydrocarbons: nature and extent of the associated hydrocarbon-DNA reaction.

A system of cell-mediated mutagenesis is described for the study of compounds which require metabolic activation to exert their cytotoxic and mutagenic effects. This system combines BHK21 cells for metabolism of the compounds and V79 cells as targets for mutagenesis. Using the two polycyclic hydrocarbon carcinogens benzo(a)pyrene and 7-methylbenz(a)anthracene we have shown that the hydrocarbon-DNA reaction which accompanies mutagenesis in the target cell is indistinguishable from that reported to occur in vivo and in primary cell cultures. Our results also support the view that a diol epoxide metabolite is responsible for the biological activity of benzo(a)pyrene. The application of cell-mediated mutagenesis to the routine testing of suspect environmental chemicals for biological activity is discussed.

Azaguanine↗